Phage T4 growing in Escherichia coli is a powerful model system for studying mutagenesis and DNA repair. (1) One model of UV mutagenesis holds that cytosines in pyrimidine dimers deaminate rapidly, so that templated bypass of the lesion inserts adenines. However, UV'd T4 particles held at temperatures and pHs that promote deamination do not display increased mutagenesis. (2) Forward mutation in T4 is often scored by counting r mutants. These arise at five loci, two of which (rI and rV) remain poorly characterized. We are helping to locate the rI genes(s) and are systematically sequencing a region believed to contain the rV locus. (3) Phage RB69 is a relative of phage T4. RB69 DNA polymerase has diverged by 40% in amino acid sequence from T4 DNA polymerase but drives T4 replication well in complementation experiments. Measurements of mutation rates in this homeologous context reveal that the fidelity of this DNA replicase is intrinsic to the polypeptide and independent of the action of the other proteins that assist DNA replication. (4) T4 survival after DNA damage is higher at higher temperatures, and mutagenesis is lower. This difference resides in recombination repair. Tests are underway to discover the responsible genic component. (5) Several alleles in two vital genes of T4 DNA metabolism define a DNA-repair epistasis group whose mechanism remains mysterious. We are testing the residuum of known T4 UV-sensitivity mutations to see if any affect this system. Then we will attempt to generate mutations of this epistasis group in other genes of DNA metabolism, and to conduct enzymological studies.